Railway track relay circuits



April 1966 H. DUCKITT ETAL 3,246,142

RAILWAY TRACK RELAY CIRCUITS Filed July 11; 1962 2 Sheets-Sheet 1 Uolleao Ciuwea E 0 B 14 c 0 50% 100% 35 m fiase #E'mc'e Walage P 01 ll D. C.

INVENTORS Ha py Duali and Dyoaglas Al. Jones. 3

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CUE ['15 HWTORA EY April 12, 1966 DUCKlTT ETAL 3,246,142

RAILWAY TRACK RELAY CIRCUITS Filed July 11, 1962 2 Sheets-Sheet 2 D. C. w/ 9- 77 It Tpacli Uc'pcud r'ac 86066081 and Cwcaz T ansnzzien 8 625 INVENTORS Harry Due/i666 and Douglas Al. Jones BY T A 27E flTY OBA EF United States Patent 3,246,142 RAILWAY TRACK RELAY CIRCUITS Harry Duckitt and Douglas M. Jones, London, England, assignors to Westinghouse Brake and Signal Company Limited, London, England Filed July 11, 1962, Ser. No. 209,030 Claims priority, applicationGre-at Britain, July 14, 1961,

1 Claim. (Cl. 246-34) This invention relates to railway track relay circuits and relates especially to relay circuits for use with noninsulated track circuits, the eifective boundaries of which are determined by the characteristics of the track and relay and the transmitter output impedance.

It is a principal object of our invention to provide track relay apparatus which more, clearly defines the limits of non-insulated track circuits.

Another object of our invention is improved relay circuit apparatus, for track circuits in non-insulated track, which provides positive closing of the relay contacts in either position with less, time delay.

According to the present invention, there is provided a track relay circuit having a relay which is coupled to the input terminals of the circuit through an amplifier, the characteristics of which are so chosen that up to a predetermined magnitude, with increase of magnitude of signal at said input terminals, a relatively low rate of increase of signal magnitude applied to said relay is produced and beyond said predetermined magnitude, a relatively high rate of increase of signal magnitude applied to said relay is produced.

In a preferred embodiment of the invention, the desired characteristics of the amplifier are obtained by employing a transistor as an alternating current amplifier and making use of the bottom bend effect of the base emitter diode of the transistor.

In order that theinvention may be clearly understood and readily carried into effect, the same will be further described with reference to the accompanying drawings, in which:

FIGURE 1 illustrates a typical characteristic curve of relay voltage for a non-insulated track circuit.

FIGURE 2 illustrates a circuit arrangement embodying one form of the invention.

FIGURE 3 illustrates a characteristic curve associated with a transistor amplifier as employed in the circuit of FIGURE 2.

FIGURE 4 illustrates the circuit arrangement of one suitable form of demodulator which may be used in conjunction with the relay circuit illustrated in FIGURE 2.

FIGURE 5 shows schematically in block diagram form a simple non-insulated track circuit in which the relay circuit arrangement of FIGURE 2, with or without the demodulator of FIGURE 4, may be used.

Prior to the further description of the invention it will be of assistance to appreciate the requirements of a track relay-circuit forv use with non-insulated track circuits. In a track circuit having block or insulated joints, that is, in which track circuitsare electrically insulated from oneanother, a sudden change in the voltage applied to the relay occursas a rail vehicle enters or leaves the by the vehicle is only effective inside the physical limits 3,246,142 Patented Apr. 12, 1966 of the respective insulated block. However, a non-insulated track circuit does not have specific physical limits and the rate of change of voltage applied to a track relay with vehicle displacement is dependent upon the characteristics of the track and relay circuits. A type noninsulated track circuit is shown in block form in FIG- URE 5. It is to be noted that the rails x and y of the track are not broken by insulated joints and are thus electrically continuous throughout. The track circuit transmitter is connected across the rails at one location and supplies to these rails an alternating current having a preselected frequency. Depending upon the type signal system, the alternating track current may be coded or non-coded. The track circuit receiver and/or relay apparatus, shown in detail in FIGURE 2, is connected across the rails at another location. Specifically, terminals P and Q of the receiver/relay circuit are each connected to a different rail. Nominally, the resulting track circuit extends between the rail connections of the transmitter and receiver and the circuit is responsive to a train occupying this section of the track. However, in practice, since there are no physical limits such as insulated joints, the track circuit apparatus and particularly the relay further responds to rail shunts within some relatively fixed distance in each direction outside the section defined by the rail connections. A specific example of prior art non-insulated track circuits and associated apparatus is shown in Letters Patent of the United States No. 3,035,- 167 issued May 15, 1962, to P. H. Luft for a Railway Track Circuit.

The response of, the track relay in the typical circuit of FIGURE 5 is illustrated in FIGURE 1, which shows a characteristic variation of relay voltage for vehicle position beyond the rail connections when leaving or entering the track circuit. The point A represents the position at which the relay picks up upon the vehicle leaving the track circuit, C represents the position at which full compression of front contacts is produced thereafter, and B represents the position at which dropaway occurs as the vehicle enters the track circuit. It is observed that, particularly in the case of a slow vehicle, a substantial time delay may occur before the relay, having picked up at A, attains full compression at C with a vehicle leaving the track circuit, and similarly, between the opening of the front contacts and compression of the back contacts with a vehicle entering the track circuit. During this period, there is a possibility of the relay contacts being intermittently broken due to a slight vibration. It is therefore desirable that the relay should go as nearly as possible to full compression as soon as the contacts are made on pick up or release. Furthermore, the limits or boundaries of the track circuit are mainly dependent upon the attenuation of a signal current flowing in the track on either side of the section under consideration and are relatively poorly defined. By choosing the characteristics of the amplifier in a track relay circuit in accordance with the invention, it is arranged that the boundaries of the track circuit as represented by the operation of a track relay are more clearly defined and the operation of the circuit more nearly approaches that of a track circuit provided with insulated joints.

Referring to FIG. 2, the circuit arrangement shown illustrates a track relay circuit according to the invention incorporated as an audio frequency receiver suitable for use in connection with steam and diesel railway territories. The circuit consists basically of a two stage band pass filter formed by capacitor C1, an inductor L1 having a secondary Winding which feeds capacitor C2 and another inductor L2, which has a secondary winding whose output is applied to the base-emitter circuit of a transistor TRl. The primary winding of a transformer T1 is included in the collector circuit of TRl. The secondary winding of transformer T1 is connected through a diode D1 to the coil RC of the track relay. A capacitor C3 is connected across the relay coil to provide a degree of smoothing.

in operation of the track circuit, the receiver input tertnin'als P and Q are connected across the track rails (as shown in FIGURE 5) and the band pass filter prevents current of any frequency other than that supplied by the transmitter associated with the track circuit from operating the relay. Assuming initially that the track circuit is shunted by a vehicle, transistor TRll is substantially nonconducting, there is virtually no direct current output across coil RC, and the relay is in a deenergized state. As the vehicle leaves the point at which P and Q are connected across the rails or the connections at which the associated transmitter is connected to the track, the voltage across terminals P, Q rises. *Until the base-emitter voltage for transistor TRl reaches a predetermined magnitude, Which for a silicon transistor may be about 0.4 volt, no increase of collector current in transistor TRI occurs. Further increase of voltage across terminals P, Q beyond the predetermined value causes the collector current to rise in the manner illustrated in FIGURE 3. From this it is seen that up to a predetermined baseemitter voltage represented by the point E, a relatively low rate of increase of signal magnitude applied to the relay is produced. Beyond this predetermined voltage, a relatively high rate of increase of signal magnitude applied to said relay is produced.

FIGURE 3 shows a typical curve of collector current against base-emitter voltage for a silicon transistor with the coordinates graduated in percentages. Points A and B represent, respectively, the pick up and drop away points for the relay. The collector current is proportional to the relay energizing current and the base-emitter Voltage is proportional to the track voltage at terminals P and Q. Since the input voltage has only to be reduced by about to change the voltage applied to the relay coil by 50%, an overall'characteristic of about 90% release to pick up voltage at the input is provided for. This reduces the inherent difficulty with certain relays that, in order to have the desirable feature of going to the full compression condition as soon as the contacts are made on pick up, a deterioration in the percentage release of the relay mustbe tolerated. For the sake of comparison, the point B is represented also on the relay characteristic shown in FIGURE 1, which makes it immediately clear than, with the proposed track relay circuit employed in conjunction with a relay which travels straight up to full compression on pick-up, the spread between release and full compression, that is points B and C, is substantially reduced.

For the sake of completeness, the case in which a track relay circuit such as shown in FIGURE 2 is used in an electrified territory will now be discussed with reference to FIGURE 4. In direct current traction territory, capacitor C1 of FIGURE 2 acts to block the direct current voltage on the track. However, harmonics developed in both direct and alternating current traction supplies may,

1 if they are of sufficiently high level, pass through the filter and cause false relay operation. To overcome this a modulated or coded audio frequency track circuit may be used and it is then necessary to provide the relay cirsuit with demodulation or decoding means.

Referring to FIGURE 4, terminals M and N are con- 4 nected to the correspondingly referenced points in the final relay connection of FIGURE 2, the connections of coil RC being moved to the output side of the demodulator. Terminals M and N are connected to the baseemitter circuit of a transistor TRZ, the emitter of which is also connected to the base of yet another transistor TR3. The emitter circuit of transistor TR3 includes a series tuned circuit formed by capacitor C4 and inductor L3, the latter being provided with a secondary winding which has its end terminals connected through diodes D2 and D3 to one terminal of the relay coil RC and has its center tap connected to the other terminal of coil RC.

To provide a discharge path for capacitor C4, a resistor R3 is connected across the tuned circuit and to prevent excessive leakage current, resistors R1 and R2 act as a tie between the base circuits of transistors TR2 and TR3.

With a modulated input to the receiver, a rectified voltage varying from V volts to zero at the modulation frequency appears across capacitor C3 (FIGURE 2) and thus across terminals M and N (FIGURE 4). When its base is at V volts, transistor TR2 conducts, providing a negative drive to the base of transistor TR3 which therefore also conducts to charge capacitor C4. When the voltage on the base of transistor TRZ tends towards zero, transistors TRZ and TR3 become out off and capacitor C4 discharges via resistor R3 and inductor L3. This repeats foreach cycle of the modulated signal frequency and provides an approximately sinusoidal voltage waveform across the secondary winding of inductor L3, which in turn is rectified by diodes D2 and D3 and applied to coil RC of the track relay.

Although the actual frequencies of signals employed with track circuit-s employing a track relay circuit in accordance with the invention have not been discussed, an audio frequency in the range 1 to 10 kc./s. may be suitable. In the case, in electrified territory, of using an arrangement such as described with reference to FIGURE 4, modulation frequencies in the range of l to 45 c./ s. may be employed. This does not however, preclude the use of frequencies outside those ranges.

Although the present invention has been discussed primarily in relation to audio frequency track circuits in areas having continuous rails, the invention may equally well be applied to track relay circuits for use in conjunction with overlay track circuits. 7

Having thus described our invention what We claim is:

A track circuit for a stretch of railway track with electrically continuous rails, comprising in combination,

(a) transmitting means with connections to the rails at a selected location for supplying thereto alternating current of a selected frequency and modulated at a preset code rate,

(b) a track relay responsive to a present level of energizing current for closing its front contacts at full compression,

- (0) a silicon transistor amplifier means responsive to periodic input signals and biased for supplying an output current of similar modulation andrapidly increasing in magnitude only when said input signals exceed a preselected energy level,

(1) the silicon transistor having the characteristic that said preset current level for said relay is reached with only a relatively small increase in magnitude of said input signal in excess of said preselected energy level,

(d) filter means connecting said amplifier means to said rails at a location different from said transmitting means rail connections for supplying as input signals to said amplifier means rail current of said selected frequency only,

(1) said rail current supplied exceeding said pre set energy level only when the track between the rail connections of said transmitting means and said filter means and a relatively fixed dis- 3,246,142 5 6 tance beyond such connections in each direc- References Cited by the Examiner tion is free of occupancy by a train, (e) a demodulator means responsive to periodic input UNITED STATES PATENTS signals at any code rate for producing a continuous 2,987,653 6/1961 Prapis 17148.5 X

current and having connections for supplying said 5 3,035,167 5/ 1962 Lllft 246-130 continuous current to energize said relay, 3,075,129 1/1963 Danemagne 48 (f) circuit means connecting said amplifier means and 81179 7/1963 Van Rossum 6t said demodulator means for supplying the output 317143-5 X current from said amplifier means as input signals EUGENE BOTZ Primary Examine"- to said demodulator means. SAMUEL BERNSTEIN, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,246,142 April 12, 1966 Harry Duckitt et a1 It is hereby certified that error appears in the above numbered petent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 53, for "present" read preset Signed and sealed this 31st day of January 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

